Downy mildew (Peronospora farinosa f. sp. spinaciae) is a major threat for spinach growers because it directly affects the harvested leaves. In spinach, downy mildew is caused by the oomycete Peronospora farinosa f. sp. spinaciae (formerly known as P. effusa). Infection makes the leaves unsuitable for sale and consumption, as it manifests itself phenotypically as yellow lesions on the older leaves, and on the abaxial leaf surface a greyish fungal growth can be observed. The infection can spread very rapidly, and it can occur both in glasshouse cultivation and in soil cultivation. The optimal temperature for formation and germination of P. farinosa f. sp. spinaciae spores is 9 to 12° C., and it is facilitated by a high relative humidity. When spores are deposited on a humid leaf surface they can readily germinate and infect the leaf. Fungal growth is optimal between 8 and 20° C. and a relative humidity of ≥80%, and within 6 and 13 days after infection mycelium growth can be observed. Oospores of P. farinosa can survive in the soil for up to 3 years, or as mycelium in seeds or living plants.
To date 16 pathogenic races of spinach downy mildew (Pfs) have been officially identified and characterized, and many new candidates are observed in the field. The 16 officially recognised races of Peronospora farinosa f. sp. spinaciae, are designated Pfs:1 to Pfs:16 (Irish et al. Phtypathol. Vol. 98 pg. 894-900, 2008; Plantum NL (Dutch association for breeding, tissue culture, production and trade of seed and young plants) press release, “Benoeming van Pfs: 14, een nieuwe fysio van valse meeldauw in spinazie”, Sep. 19, 2012; Report Jim Correl (Univ. Arkansas) and Steven Koike (UC Cooperative Extension, Monterey County), “Race Pfs: 14—Another new race of the spinach downy mildew pathogen”, Sep. 18, 2012; Plantum NL press release, “Denomination of Pfs: 15, a new race of downy mildew in spinach”, Sep. 2, 2014, Plantum NL press release, “Denomination of Pfs: 16, a new race of downy mildew in spinach, Mar. 15, 2016). Races 4 to 15 were identified between 1990 and 2014, while only recently another new Peronospora isolate has been identified, termed UA201519B, which subsequently has been officially named Pfs:16 by the International Working Group on Peronospora (IWGP) (Plantum NL (Dutch association for breeding, tissue culture, production and trade of seed and young plants) press release, “Denomination of Pfs: 16, a new race of downy mildew in spinach”, Mar. 15, 2016. All 16 officially recognized Pfs races are publicly available from the Department of Plant Pathology, University of Arkansas, Fayetteville, Ark. 72701, USA, and also from NAK Tuinbouw, Sotaweg 22, 2371 GD Roelofarendsveen, the Netherlands.
Especially the latest identified Peronospora races can break the resistance of many spinach varieties that are currently used commercially worldwide, and they thus pose a serious threat to the productivity of the spinach industry. Therefore, it is crucial to stay at the forefront of developments in this field, as Peronospora continuously develops the ability to break the resistances that are present in commercial spinach varieties. For this reason new resistance genes against downy mildew are very valuable assets, and they form an important research focus in breeding and particular in spinach and lettuce breeding. One of the main goals of spinach breeders is to rapidly develop spinach varieties with a resistance to as many Peronospora races as possible, including the latest identified races, before these races become wide-spread and pose a threat to the industry.
In commercial spinach varieties resistance against downy mildew is usually caused by so-called R-genes. R-gene mediated resistance is based on the ability of a plant to recognize the invading pathogen. In many cases this recognition occurs after the pathogen has established the first phases of interaction and transferred a so called pathogenicity (or avirulence) factor into the plant cell. These pathogenicity factors interact with host components in order to establish conditions which are favorable for the pathogen to invade the host and thereby cause disease. When a plant is able to recognize the events triggered by the pathogenicity factors a resistance response can be initiated. In many different plant pathogen interaction systems such as the interaction of spinach with different downy mildew strains, the plant initiates these events only after specific recognition of the invading pathogen.
Co-evolution of plant and pathogen has led to an arms race in which a R-gene mediated resistance is sometimes overcome as a consequence of the capability of the pathogen to interact with and modify alternative host targets or the same targets in a different way, such that the recognition is lost and infection can be established successfully resulting in disease. In order to re-establish resistance in a plant, a new R-gene has to be introduced which is able to recognize the mode of action of an alternative pathogenicity factor.
Despite the fact that the durability of R-genes is relatively low, R-genes are in spinach still the predominant form of defense against downy mildew. This is mainly due to the fact that it is the only form of defense that gives absolute resistance. So far plant breeders have been very successful in generating downy mildew resistant spinach varieties by making use of resistance genes residing in the wild germplasm of the crop species. Even though R-genes are extensively used in spinach breeding, until now not much is known of these R-genes. The R-genes present in the current commercial spinach varieties have never been characterized at the molecular level, i.e. their sequence until now was unknown. Also up until now there are no closely linked molecular markers known in the art that separate these R-genes, nor are the molecular characteristics of the genes themselves known in the art. Therefore, the search for new R-genes and R-gene identification is currently based on phenotypic assays in which many accessions are screened for possible variation in their resistance pattern. Subsequently it has to be determined through crossing and selection whether a newly observed resistance is in fact caused by an R-gene.
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.